C3 and C4 Biomass Allocation Responses to Elevated CO2 and ...

Estuaries and Coasts (2012) 35:1028–1035 DOI 10.1007/s12237-012-9500-4

C3 and C4 Biomass Allocation Responses to Elevated CO2 and Nitrogen: Contrasting Resource Capture Strategies K. P. White & J. A. Langley & D. R. Cahoon & J. P. Megonigal

Received: 24 March 2011 / Revised: 17 February 2012 / Accepted: 20 March 2012 / Published online: 1 May 2012 # Coastal and Estuarine Research Federation 2012

Abstract Plants alter biomass allocation to optimize resource capture. Plant strategy for resource capture may have important implications in intertidal marshes, where soil nitrogen (N) levels and atmospheric carbon dioxide (CO2) are changing. We conducted a factorial manipulation of atmospheric CO2 (ambient and ambient+340 ppm) and soil N (ambient and ambient+25 gm−2 year−1) in an intertidal marsh composed of common North Atlantic C3 and C4 species. Estimation of C3 stem turnover was used to adjust aboveground C3 productivity, and fine root productivity was partitioned into C3–C4 functional groups by isotopic analysis. The results suggest that the plants follow resource capture theory. The C3 species increased aboveground productivity under the added N and elevated CO2 treatment (P< 0.0001), but did not under either added N or elevated CO2 alone. C3 fine root production decreased with added N (P< 0.0001), but fine roots increased under elevated CO2 (P0 K. P. White Departments of Forest Ecology and Biogeosciences, and Statistical Science, University of Idaho, PO Box 83844-1133, Moscow, ID 83844, USA J. A. Langley : J. P. Megonigal (*) Smithsonian Environmental Research Center, PO Box 28, 647 Contees Wharf Road, Edgewater, MD 21037, USA e-mail: [email protected] J. A. Langley (*) Biology Department, Villanova University, 800 Lancaster Ave, Villanova, PA 19085, USA e-mail: [email protected] D. R. Cahoon US Geological Survey, Patuxent Wildlife Research Center, 10300 Baltimore Avenue, BARC—East Building 308, Beltsville, MD 20705, USA

0.0481). The C4 species increased growth under high N availability both above- and belowground, but that stimulation was diminished under elevated CO2. The results suggest that the marsh vegetation allocates biomass according to resource capture at the individual plant level rather than for optimal ecosystem viability in regards to biomass influence over the processes that maintain soil surface elevation in equilibrium with sea level. Keywords Biomass . Chesapeake Bay . Productivity . Sea level rise . Tidal marsh . Turnover

Introduction Plants can optimize resource capture through shifts in biomass allocation under differing resource environments, and these shifts follow conserved patterns. Specifically, a tradeoff exists between producing aboveground structures for light capture and belowground structures for soil resource capture (Tilman and Wedin 1991; Craine 2009). Such patterns have been reported in ecosystems as diverse as grasslands (Suter et al. 2002) and forests (Pregitzer et al. 1995), and they tend to vary predictably across species according to photosynthetic functional group (Reynolds and Dantonio 1996). The rules of resource capture and biomass allocation have especially important consequences in intertidal marsh ecosystems. Here, vegetation plays a direct role in sustaining the ecosystem. In tidal marshes, the presence of vegetation allows the ecosystem to maintain an equilibrium elevation relative to sea level (Redfield 1965; Morris 2006) by two primary mechanisms: (1) accumulation of organic soil by deposition of mostly endogenous organic matter (Turner 2004; Nyman et al. 2006; Mitsch and Gosselink 2007) and (2) trapping of exogenous sediments during tidal inundation (Morris et al.

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2002; Mudd et al. 2010). Shifts in biomass allocation under an altered resource environment may alter these processes, and hence alter the viability of the marshes (Reed 1995; Morris et al. 2002). Soil nitrogen (N) availability commonly limits marsh productivity (Valiela and Teal 1974) and is one of the more important resources affecting plant allocation. Under enhanced soil N availability, marsh plants may allocate more biomass aboveground, where aerobic decomposition decreases the amount of organic matter delivered to the soil. In turn, the shift in allocation aboveground may decrease the ability of the marsh to gain soil surface elevation relative to sea level where organic matter accumulation is most important to soil accumulation and hence marsh viability (Langley et al. 2009a). On the other hand, increases in atmospheric CO2 often stimulate belowground production more than aboveground (Rogers et al. 1994; Iversen et al. 2008), but the stimulation usually depends on the photosynthetic functional group of the species (Poorter and Navas 2003). It remains unknown how large-scale perturbations such as rising CO2 and nitrogen pollution will alter allocation patterns among marsh plants, which will have critical influence on plant-mediated mechanisms of soil elevation gain. We conducted an experiment in a brackish tidal marsh to examine how plants alter plant biomass allocation patterns under combinations of enhanced soil N availability and increased atmospheric CO2. To do this, we estimated biomass and productivity of C3 and C4 vegetation under factorial treatments of ambient and high soil N and atmospheric CO2. Previous work reported earlier treatment effects on plot-level productivity (Langley et al. 2009a; Langley and Megonigal 2010); however, in this study, accounting for alteration in C3 stem turnover allowed more accurate estimation of biomass allocation aboveground. Biomass allocation belowground is defined between the functional C3–C4 groups through carbon isotope analysis, allowing us to distinguish the strategies of these two functional groups that differ dramatically in photosynthetic strategy but commonly co-occur in brackish wetlands. Moreover, we take advantage of aboveground measurements of C3 species in order to understand how changes in soil N availability may alter the ability of the marsh to trap sediments.

Site Description and Methods Study Site The study site is located in Kirkpatrick Marsh, the Smithsonian Institution’s Global Change Research Wetland. Kirkpatrick Marsh is a high, brackish, intertidal marsh on the Chesapeake Bay in Maryland, in the USA (38°53′ N, 76°33′ W). The study plots were composed entirely of three species: a C3 sedge,

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Schoenoplectus americanus (formerly Scirpus olneyi), and two C4 grasses, Spartina patens and Distichlis spicata. The soils of the marsh are histosols composed of more than 80 % organic matter. Active rooting by the marsh vegetation takes place in the upper 50 cm of the soil profile (Saunders et al. 2006). Directly below the active rooting zone, a more highly decomposed layer extends to a depth of 4–5 m where it meets a clayey mineral horizon. Salinity in the marsh is relatively low compared to other study systems, ranging from 4 to 15 ppt depending on regional rainfall. Tidal inundation of the marsh occurred in about 15 % of high ocean tides. More details of the site are given in Langley et al. (2009b). Experimental Design The open-top chambers used in this study were a modification of an earlier chamber design that had been rigorously tested for in situ elevated CO2 experimentation (Drake et al. 1989). A chamber consisted of an octagonal frame and frustum, mounted on top of a hollow manifold for distributing air, which was mounted on an aluminum base that extended 30 cm into the soil. Each chamber enclosed 3.3 m2 of marsh area. In this study, 20 of the chambers were installed in total. Detailed information about the chambers can be found in Langley et al. (2009a, b). There were four experimental treatment groups consisting of atmospheric CO2 and soil N additions in a 2×2 factorial design. Elevated atmospheric CO2 was delivered in one of two levels, either ambient (i.e., no added atmospheric CO2) or elevated CO2 (ambient+340 ppm). Ten of the chambers received ambient CO2. The other ten received the elevated CO2 during daylight hours and ambient CO2 during night. The CO2 delivery occurred throughout each of two growing seasons, from late April through early November. The concentration of CO2 inside each chamber was logged at 40-min intervals to document the precision of the elevated CO2 treatments (Langley et al. 2009b). Like CO2, N was added in one of two levels, either no enhancement, or at a rate of 25 g N m−2 year−1. The enhanced N level was applied to ten of the chambers in 5-g N m−2 applications monthly from May through September each year. N was applied by spraying NH4Cl (equivalent to 25 g N m−2 year−1) dissolved in local brackish water evenly across the plots. Following N application, an equal volume of unamended brackish water was sprayed onto the plots to rinse the N solution from the vegetation canopy to the soil surface. The enrichment in soil N availability was confirmed by elevated porewater [N] in the root zone, averaging increase over controls by 25 % over the course of the experiment (Langley and Megonigal 2010). The ten chambers that were to receive no N enhancement were sprayed with unamended brackish water in the same volume as the N enhanced chambers. The four treatments were (1) ambient CO2 without N addition (Amb), (2) ambient CO2

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with N addition (Amb+N), (3) elevated CO2 without N addition (Elev), and (4) elevated CO2 with N addition (Elev+N). The experimental design was balanced (n 05 for each treatment). Plant Growth Measurements S. americanus aboveground biomass and productivity were estimated by a combination of annual harvest subsampling and tracking of stem turnover during the growing seasons. The harvests were undertaken in late July of each year by a method modified from Curtis et al. (1989). During the harvest, a subsample of S. americanus stems was measured, clipped, oven-dried at 60 °C for 72 h, and then weighed. The subsample measurements and weights were used to develop allometric equations (r2 >0.9 in both years) that were then used to estimate biomass during a larger, nondestructive sampling. The nondestructive sampling of S. americanus was conducted by measuring stem density, height, and width of shoots within six 30-cm2 quadrats within each chamber. The area sampled totaled 16.3 % of the area within each chamber. The nondestructive measurements and allometric equations were used to estimate C3 standing biomass for each season. We estimated C3 shoot turnover by tracking a subsample of individually tagged S. americanus stems and monitoring the number of stems that senesced prior to harvest (N0200 in 2007 and N0120 in 2008). C3 shoot productivity was then estimated as follows: C3 above ground productivity ¼ C3 peak standing biomass þ ½ðfraction C3 dead at peakÞ ðC3 peak standing biomassÞ

and 30 cm in height. Each winter, in each chamber, three ingrowth bags were filled with root-free peat, inserted into precored holes, and retrieved the following winter. The functional group (C3 or C4) of coarse roots (>2 mm diameter) and rhizomes was determined by obvious visual characteristics (Saunders et al. 2006). For fine roots, visual distinction between the functional groups was not possible. Fine roots were separated into C3 and C4 functional groups using naturally occurring differences in δ13C (Saunders et al. 2006). To measure the isotopic differentiation, the fine roots in the samples were ground and analyzed for 13C composition at the UC Davis Stable Isotope Laboratory using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). A mixing model was used to partition the fraction of fine root growth from S. americanus and the C4 grasses as follows: Fraction of fine root C from C3 source ¼ d 13 Cfineroot d 13 CC 4 þ a = d 13 CC3 d 13 CC 4 where δ13Cfine root is the 13C composition of a homogenized single fine root sample composited from the three replicate ingrowth bags from each plot. d 13 CC3 and d 13 CC 4 represent foliar 13C for each functional group, determined separately for each plot to account for plot-level variability in strength of the novel 13C signature. An offset (α) was used to account for differences between shoots and roots arising either from fractionation or the incomplete integration of novel 13C into the root system (Langley et al. 2002). The α value was determined by comparing roots from a known species to shoots in the same plots for each CO2 treatment group in each year (2007: ambient0−1.0‰, elevated0−0.5‰; 2008: ambient0 −0.5‰, elevated01.1‰). Trapping Index

where the fraction dead was the number of dead C3 stems as a proportion of all stems (live + dead) over the course of the growing season. Grass aboveground biomass (C4 shoot biomass) was also estimated by measurements collected during the annual harvests; however, allometry was not used owing to the intractably large number of individual stems (Curtis et al. 1989). Instead, at each harvest, six 5-cm2 areas were clipped from each chamber. The subsamples were oven-dried at 60 °C for 72 h and weighed, and then, those weights were scaled to the plot level as the estimates of C4 shoot biomass. Though commonly used as a proxy for productivity, peak C4 biomass underestimates productivity because shoot turnover and new growth after the peak biomass harvests were not measured. Annual root productivity (C3 and C4 fine root, coarse roots, and rhizome) was estimated by weighing ovendried, new root growth recovered from subsurface, mesh ingrowth bags. The bags were cylindrical, 5 cm in diameter,

To estimate how the treatments may alter the plant effect on sediment trapping of S. americanus, we calculated a sediment-trapping index for each of the treatments. The index is based on plant stem density (Leonard et al. 1995; Leonard and Luther 1995) and stem width and height (as suggested by the findings of Palmer et al. 2004). The index was calculated simply as the product of C3 stem density, height, and width. Rather than predicting actual sediment accumulation at this sediment-poor site, this value was used to assess the potential for the global change treatments to affect sediment-trapping potential of one species. Statistical Analyses C3 and C4 standing biomass, C3 shoot productivity, C3 and C4 coarse root and fine root productivity were analyzed separately. Because coarse roots and rhizomes showed


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highly variable masses, rhizomes were included in “coarse roots” for analysis. We also analyzed aboveground plant parameters C3 stem density, height and width, as well as the calculated trapping index. Before any modeling, univariate analysis on each of the analyzed response variables indicated normality (Shapiro–Wilks, P>0.05), so data transformation was unnecessary. Initially, the data were analyzed in MANOVAs with year as the within group variable. However, the year was generally insignificant, and at no point interacted with other factors, so the data over the 2 years were averaged for each plot. We followed by performing individual ANOVAs on each response. Fisher’s LSD multiple comparisons were conducted post hoc.

Results C3 aboveground biomass and productivity were both influenced by elevated CO2 and enhanced N addition (Table 1). C3 standing biomass decreased from the control when only N was added (Amb>Amb+N, P00.0423), but when N was added in combination with elevated CO2, the C3 standing biomass increased (Elev+N>Amb, P00.0034; Fig. 1a). Accounting for turnover in the Amb+N and Elev+N treatments increased C3 standing biomass estimates by 20–30 % (Fig. 1b; Table 2). Consequently, C3 shoot productivity within treatments only differed from the control under the Elev+N treatment (Elev+N>Amb, PAmb, PAmb, P00.0214), but the gain was less than under the N only treatment (Amb+N>Elev+N, P00.0002). Belowground C3 productivity was influenced differently by the treatments, depending on the particular root diameter class (Table 1). C3 coarse root productivity did not change from the control under any of the treatments (Fig. 3a). There Table 1 P values from two-way ANOVAs evaluating the treatment effects on C3 and C4 growth

Standing biomass

C3

Shoot productivity

C3 C4 C3 C4 C3 C4

Coarse root productivity Fine root productivity

P values